1. Field of the Invention
The present invention relates to particular multiple purpose catalysts having enhanced mechanical properties, and, more especially, to such catalysts particularly adopted for the treatment of the exhaust gases emanating from internal combustion engines.
By the expression "multiple purpose catalysts" as utilized herein, are intended those catalysts useful, in particular, for the oxidation of carbon monoxide and the hydrocarbons present in vehicular exhaust gases, as well as for the simultaneous reduction of the nitrogen oxides (NO.sub.x) present therein.
2. Description of the Prior Art
In internal combustion gasoline engines, the compositions of the exhaust gases may be controlled around a stoichiometric equilibrium, such as by the catalytic oxidation and reduction of the different components thereof, thus producing water, carbon dioxide and nitrogen. The means typically employed to control the composition of exhaust gases at about stoichiometric equilibrium includes, in particular, the continuous regulation of the air/fuel ratio at the engine intake and/or the introduction of additional oxygen upstream from the catalyst. The composition of the exhaust gas thus varies over periods of time on the order of one second, from a composition containing a relative excess of oxidizing compounds (designated "lean"), to a composition containing an excess of reducing compounds (designated "rich"), and vice versa.
In particular, the regulation designated "lean" is such that the amounts of oxygen and nitrogen oxide present are higher than those required for the total oxidation of carbon monoxide, the hydrocarbons and hydrogen. Conversely, the regulation designated "rich" is such that the amounts of carbon monoxide, hydrocarbons and hydrogen present are higher than those required for the total reduction of the oxygen and nitrogen oxides.
A great number of catalysts have already been proposed to this art for the total elimination, simultaneously, of the nitrogen oxides, unburned hydrocarbons and carbon monoxide contained in the exhaust gases of internal combustion engines. These catalysts typically contain precious metals, possibly combined with a base metal, such as nickel, iron, zirconium, vanadium, chromium or cerium, deposited onto a conventional refractory oxide support.
Thus, European Patent No. 27,069, assigned to the assignee hereof, describes a catalyst comprising a refractory oxide support and an active phase of cerium, iron, at least one metal such as platinum or palladium, and at least one metal such as iridium or rhodium.
According to German Patent No. 3,233,500, multiple purpose catalysts comprising an active phase of rhodium, palladium, iron and cerium, are also known.
In French Patent No. 2,568,143, also assigned to the assignee hereof, multiple purpose catalysts are described having rhodium, palladium, iron, cerium and zirconium active phases.
These catalysts, although they indeed possess sufficient activity to satisfy the anti-pollution standards adopted in countries such as the United States and Japan, have the disadvantage in that a portion of their multi-purpose activity diminishes over time when used in an automotive catalytic converter.
A further disadvantage of the catalysts of the prior art resides in the fact that, when used in particle form (spheres, tablets, extrudates, pellets, etc.), specifically in catalytic converters for exhaust gases, their mechanical properties, and especially their resistance to wear and attrition, are not always completely satisfactory.
The consequences of such mechanical wear relative to the catalytic performance of catalysts of this type are apparent. Their economical use is also adversely affected.
Therefore, mechanical wear constitutes a significant additional problem, especially when superimposed on the aforementioned problem of maintenance of the multiple purpose activity of these particular catalysts.